Changing Patterns of New Tuberculosis Infections

Abstract & Commentary

Synopsis: Although the incidence of tuberculosis has declined in the United States in the last decade, new cases have actually risen among foreign-born residents. Molecular and epidemiological analyses of tuberculosis case isolates in New York City suggest that reactivation of latent infection is responsible for the rise, while declining rates of acute transmission account for most new cases in US-born residents.

Source: Geng E, et al. Changes in the transmission of tuberculosis in New York City from 1990 to 1999. N Engl J Med. 2002;346:1453-1458.

Starting with 812 new cases of tuberculosis (TB) diagnosed between 1990-1999 at Columbia Presbyterian Medical Center in northern Manhattan, and eliminating 29 who were not local residents, 575 isolates (77%) were able to be "DNA fingerprinted" using restriction-fragment-length polymorphism. This enabled clusters of cases from recent transmission to be distinguished from unique cases, and further information on clinical, social, and demographic variables could then be linked from records at the Tuberculosis Control Program in the New York City Department of Health.

Unique TB isolates were found in 52% of the cases, while the remaining 48% were part of clusters, with matching of at least one other in the cohort, implying they were caused by recent transmission. However, this pattern changed with time over the 10-year study period, such that clustered isolates initially accounted for 63% but declined to 31% by 1999. By the end of the decade, most new cases of TB in this area were caused by unique strains.

Population characteristics associated with recent clustered transmission were (in descending order) injection-drug use, homelessness, black race, pulmonary source of isolate, HIV infection, and male sex. For unique isolates presumably arising from latent reactivation, the most likely characteristics were Hispanic ethnic background, diagnosis after 1993, non-US birth, white race, age older than 60 years, and Asian race.

Using a multivariate analysis, the strongest independent association for reactivation of latent infection was non-US birth with Hispanic ethnic background, diagnosis after 1993, age > 60 years, and other non-US births. The other characteristics found associated with recent transmission did not identify any greater risks in the non-US-born group, except for HIV infection that increased the risk of a clustered or recently transmitted infection rather than a latent reactivation.

Comment by Mary Elina Ferris, MD

These results have important implications for TB control programs in areas of recent immigration. The extremely high rate of new TB cases in this New York City neighborhood at > 125 cases/100,000 persons was far higher than the city-wide rate of 50/100,000, and much higher than the US overall rate of 10.5/100,000 in 1992. With one of the highest concentrations of recent immigrants in Manhattan (40% non-US born, mostly from the Dominican Republic), Geng and colleagues hypothesized that this foreign-born population was either not being reached by TB control efforts or else had different sources of infection. The hospital also served an adjacent area of US-born African-Americans that could be used as a comparison group.

Control of TB in New York City was remarkably successful from 1992-2000, resulting in a decline of 65% in new cases. Improvements were attributed to directly observed therapy, infection control, and standardization of initial drug treatment regimens.1 However, it soon became apparent that new cases at the end of the decade were predominantly in non-US born residents. This study shows that most of these more recent infections resulted from reactivation of latent TB, which would not necessarily be affected by the measures that reduced new acute transmissions of TB.

New immigrants to the United States are required to be free of active TB on chest x-ray, but there are currently no specific requirements for skin testing that could detect latent disease. TB remains a serious problem worldwide, accounting for 2-3 million deaths annually, and in countries with limited health facilities, up to a 36% infection rate. The Institute of Medicine recommended intensified interest in latent TB 2 years ago,2 predicting that soon the majority of new cases of TB in the United States would occur in foreign-born persons coming from nations with high rates of the disease. Mexican immigrants, for example, currently account for nearly 25% of all new cases in the United States

Clinicians should be aware that the risk of reactivating latent TB for immigrants is highest during the first 5 years after arrival; of those who develop the disease, a third do so within 1 year. Current guidelines from the CDC and others encourage skin testing of recent immigrants, and 9 months of isoniazid treatment for latent infection, for persons from countries of high tuberculosis prevalence.3,4 Until Immigration regulations and public health efforts change to target these populations, the burden for detection of these new tuberculosis cases remains in our own primary care offices. 

Dr. Ferris is Clinical Associate Professor, University of Southern California, Los Angeles, Calif.

References

1. Frieden TR, et al. N Engl J Med. 1995;333:1453-1458.

2. Geiter L, ed. Ending Neglect: The Elimination of Tuberculosis in the United States Washington DC: National Academy Press; 2000:292.

3. American Thoracic Society, CDC. Am J Resp Crit Care Med. 2000;161:S221-S247.

4. CDC. MMWR Morb Mortal Wkly Rep. 2001;50(No. 34):733-736.